1. Field of the Invention:
This invention relates to a process for producing a bulk permanent magnet such as one used in a compact motor with high output power, and more particularly, it relates to a process for producing a bulk permanent magnet directly from a melt spun powder of a rare earth-iron-boron material. The resulting bulk permanent magnet has an excellent demagnetizing force which is resistant to a strong demagnetizing field derived from an armature reaction. The bulk permanent magnet also has a high coercive force and a high residual induction which is concerned with an improvement in the output power of motors. According to the process of this invention, bulk permanent magnets having such excellent characteristics can be produced with high dimensional precision and high productivity.
2. Description of the Prior Art:
A permanent magnetic material in the non-equilibrium state or a metastable permanent magnetic material can be obtained by rapid solidification of a rare earth-iron-boron material with a melt spinning technique to solidify at least one part of the melted alloy without causing its crystallization. It is known that the resulting permanent magnetic material has a high coercive force and a high residual induction due to its non-equilibrium or metastable state (Japanese Laid-open Patent Publication No. 59-64739). However, because the permanent magnetic material obtained by such a melt spinning technique is a powder in the form of thin ribbon or flake, it must be fused by a certain method to form a bulk permanent magnet such as one used in a motor.
Examples of the method for fusing a melt spun powder include a powder metallurgy such as a non-pressure sintering process. However, when a melt spun powder of a rare earth-iron-boron material is sintered without applying pressure, excellent magnetic characteristics based on the non-equilibrium or metastable state may be degraded.
To solve this problem, a method for fusing a melt spun powder by plastic deformation has been proposed. This method comprises the steps of: charging a melt spun powder of a rare earth-iron-boron material into the cavity of a graphite mold; fixing the melt spun powder by hot pressing with an induction heating system, thereby causing the plastic deformation of the melt spun powder together with the diffusion of atoms at the interface between the adhered powder particles, to form a bulk permanent magnet (Japanese Laid-open Patent Publication No. 60-100402). The degree of fixation depends on the viscosity of the melt spun powder. When a melt spun powder having a lower viscosity is used, a higher degree of fixation can be obtained. However, it is necessary to heat the melt spun powder to a temperature higher than or equal to the crystallization temperature, for example, 600.degree. C. to 900.degree. C., for the purpose of attaining a sufficient decrease in the viscosity. Usually, several hours are required for heating the melt spun powder up to such a high temperature, after charging the powder into the cavity of a mold. The heating procedure for a long period of time may lead to a decrease in the productivity. Also, because the melt spun powder reaches an equilibrium state, excellent characteristics based on the non-equilibrium or metastable state may be degraded. Moreover, when the melt spun powder is simply compressed in the cavity of a mold, a high pressure of 1 to 3 ton/cm.sup.2 must be applied in order to combine the powder particles with each other, because the surface of the powder particles does not have a low enough potential energy. Therefore, in this case, the durability of the mold will be decreased. In addition, the bulk permanent magnet prepared by the use of such a graphite mold does not have high dimensional precision. Therefore, the resulting bulk permanent magnet formed into a near net shape must be further processed by grinding.